High intensity discharge (HID) lamp, such as metal halide discharge lamps, are characterized by high efficacy and superior color rendering index, and are widely used for general lighting. Although an HID lamp has distinct advantages, it has the significant shortcoming that hot restrike or restart is very difficult and inhibits rapid on and off operation. One of the reasons for the difficulty in hot restarting is that the pressure in the arc tube remains very high when the lamp is extinguished. In the conventional HID lamp ballast, no power is applied to the lamp after the lamp is turned off. Consequently, ionization of the gas within the arc tube stops. Restarting of the lamp requires a hot restrike voltage of several tens of kilovolts.
A typical HID lamp, energized by a ballast without instant restrike circuitry, requires a delay of more than five minutes to restart the hot lamp. In many applications, the time delay during the cooling cycle is acceptable. In some applications, such as emergency lighting, projection, graphic arts and stadium lighting, as well as when the lamp is coupled to an occupancy sensor, the cooling delays are unacceptable.
Relighting of the lamp requires a hot restrike voltage of several tens of kilovolts applied to the lamp for relighting. The high voltage requirement is due to the high gas pressure, mainly mercury and metal halide salts vapor, present in the arc tube after the lamp is turned off. To restart the lamp without very high voltages, it has been necessary to wait for the arc tube to cool down such that the gas pressure decreases to a low enough value for a standard ballast to start the lamp. Use of high voltages requires special fittings or fixtures, and special insulation for power cables. Accordingly, reduction of the high voltage requirements for hot restarting of HID lamps would be beneficial for fixture manufacturers.
U.S. Pat. No. 4,455,510, issued Jun. 19, 1984 to Lesko, discloses a high intensity discharge ballast which includes a capacitive discharge network that produces initial breakdown of hot HID lamps. However, relighting of the lamp requires a hot restrike voltage of several tens of kilovolts.
U.S. Pat. No. 4,529,914, issued Jul. 16, 1985 to Kaneda, discloses a high intensity discharge lamp having an arc tube including a pair of main electrodes and at least one auxiliary electrode or probe. A high frequency, high voltage is applied to the probe for forming a high frequency ignition discharge for establishing a low frequency arc discharge between the main electrodes. Ignition and hot restrike can be achieved by applying through the outer leads a superimposed voltage including the source voltage and the high frequency, high voltage. A drawback of the disclosed lamp is the requirement for a specially designed arc tube that includes an auxiliary electrode or probe. This approach cannot be utilized with standard two-electrode discharge lamps.
U.S. Pat. No. 6,191,537, issued Feb. 20, 2001 to Celso, discloses a solid state igniter device for hot restrike of discharge lamps with an associated supply circuit. The circuit does not produce pulses after the lamp is de-energized, such that relighting of the lamp requires hot restrike voltage pulses of several tens of kilovolts.
Accordingly, there is a need for improved methods and circuits for instant hot restart of high intensity discharge lamps.